Mercruiser MPI - How Intelligent is the ECM?

[Superjetjim]

What parameters would the ECM/ECU adjust on a 4.3MPI if the thermostat was stuck open and the motor therefore running cold at around 100deg or less (rather than the usual 160ish)?

I guess it would richen the mixture, but what else like timing, reduced power as engine not up to operating temp etc.?

Also would it throw a code after a while if not reaching temp?

Thanks for your help in advance

 

[dubs283]

Won't set a fault if running cool/cold. Would be in warm up mode unless engine temp came up to 125 (+). Most likely would still run rich tho.

Can't really see timing as an issue but again that is controlled mostly by the ecm. I watched timing on data items with diacom/cds with mpi engines running and it moves all over depending on load/rpm.

I'd suggest investigating/replacing the stuck thermostat

 

[Superjetjim]

Won't set a fault if running cool/cold. Would be in warm up mode unless engine temp came up to 125 (+). Most likely would still run rich tho.

Can't really see timing as an issue but again that is controlled mostly by the ecm. I watched timing on data items with diacom/cds with mpi engines running and it moves all over depending on load/rpm.

I'd suggest investigating/replacing the stuck thermostat

Thanks, that’s exactly what I did (replace thermostat) it’s now reaching temperature correctly. I haven’t water tested it yet other than running it on the hose and all seems good now (it’s reaching temp for the first time)
It was interesting to see what I thought was stopping the thermostat closing, some debris. But have cleaned it out and doing the ‘stove hot water test’ it was stuck solid - the new one in the same pan opened and closed perfectly.

 

[Superjetjim]

Temp

 

[Lou C]

It’s a chronic problem with raw water cooled engines…